Duk-Hyun Choi

Dependence of the mechanical properties of nanohoneycomb structures on porosity

Mechanical properties of nanohoneycomb structures are measured for varying porosity (or pore diameter) of the nanohoneycomb structure. The indentation modulus and hardness in the pore direction (or thickness direction) are obtained from indentation tests using a nano-indenter. The bending modulus of the nanohoneycomb structures in the vertical direction relative to the pore (generally along the beam length) is determined from bending tests in AFM. To determine the bending modulus of the nanohoneycomb structures, the area moment of inertia of the nanohoneycomb structure is determined according to the arrangement of the pores. The indentation moduli and the hardness are found to decrease nonlinearly with increasing porosity. The bending moduli of the nanohoneycomb structures also decrease nonlinearly as a function of porosity over a large range. It is made clear that the elastic modulus of a homogenous material can be controlled by changing the pore diameter.

Structure-dependent adhesion and friction on highly ordered metallic nanopore membranes

Highly ordered metallic nanopore membranes are fabricated by direct deposition of nickel on typical porous anodic alumina (PAA) templates. The large-area uniform nanopore arrays of the PAA templates are accurately transferred to the metallic nanopore replicas, depending on the thickness of the deposited metal and the pore size of the base template. We demonstrate the ready tunability of the pore size and reproducibility of the metallic nanopore structure in a wide range of pore sizes. The adhesion and friction characteristics of the metallic replicas are studied according to the pore size using atomic force microscopy (AFM). As the pore diameter increases, the friction coefficients increase nonlinearly, and the adhesive forces scarcely change. These characteristics are understood in terms of the structural properties of the replicas, specifically the surface morphology and the real contact area. Initial pore formation from a flat thin film reduces the adhesive force by up to four times.

Improved lateral force calibration based on the angle conversion factor in atomic force microscopy

A novel calibration method is proposed for determining lateral forces in atomic force microscopy (AFM), by introducing an angle conversion factor, which is defined as the ratio of the twist angle of a cantilever to the corresponding lateral signal. This factor greatly simplifies the calibration procedures. Once the angle conversion factor is determined in AFM, the lateral force calibration factors of any rectangular cantilever can be obtained by simple computation without further experiments. To determine the angle conversion factor, this study focuses on the determination of the twist angle of a cantilever during lateral force calibration in AFM. Since the twist angle of a cantilever cannot be directly measured in AFM, the angles are obtained by means of the moment balance equations between a rectangular AFM cantilever and a simple commercially available step grating. To eliminate the effect of the adhesive force, the gradients of the lateral signals and the twist angles as a function of normal force are used in calculating the angle conversion factor. To verify reliability and reproducibility of the method, two step gratings with different heights and two different rectangular cantilevers were used in lateral force calibration in AFM. The results showed good agreement, to within 10%. This method was validated by comparing the coefficient of friction of mica so determined with values in the literature.

Fabrication of Metal Nanohoneycomb Structures and Their Tribological Behavior

Metal nanohoneycomb structures were fabricated by E-beam evaporation and a two-step anodization process in phosphoric acid. Their tribological properties of adhesion and friction were investigated by AFM in relation to the pore size of the nanohoneycomb structures. Variations of the adhesive force are not found with pore size, but formation of the pore greatly reduces the adhesive force compared to the absence of pore structure. The coefficient of friction increased nonlinearly with pore size, due to surface undulation around the pore. Tribological properties do not differ greatly between the original nanohoneycomb structure and the metal nanohoneycomb structure.

A Suggestion and a Contribution for the Improvement of Axiomatic Design

For a long time, design methodologies strive to systematize the design process in order to make the practice more efficient and effective. One of such methodologies is Axiomatic Design. However, this design theory still has some problems and is not completely settled. In this paper, the new issue for the non-linear design in Axiomatic Design is suggested and the representation of system architecture by flow chart is corrected. In the case that the design equation varies as a function of the variation of DP (DP), the design should be regarded as the nonlinear design because the design equation is not a constant. When system architectures are represented by a flow chart, all systems could not be independently represented. However, by adding the notation of the end of decoupling to the notations using in flow chart, those can be independently represented.

A New Experimental Technique for Calibration of Frictional Force in Atomic Force Microscopy

A new method has been proposed for the calibration of frictional forces in atomic force microscopy. Angle conversion factor is defined using the relationship between torsional angle and frictional signal. Once the factor is obtained from a cantilever, it can be applied to other cantilevers without additional experiments. Moment balance equations on the flat surface and top edge of a commercial step grating are used to obtain angle conversion factor. Proposed method is verified through another step grating test and frictional behavior of Mica.